Chlorobionta (Green Plants) - 2

The general characteristics of the green plants are touched on above. The purpose of this section is to introduce the prasinophytes. These are a paraphyletic group of green algae which radiate from the base of the Chlorobionta.  Most are photosynthetic flagellates. In addition, the prasinophytes are the only mixotropic plants, i.e., they obtain food both by photosynthesis and phagotrophy. This is, presumably, how they obtained chloroplasts in the first place.

The phycomate prasinophytes (those with large, thick-walled floating stages, or "phycomata") have received special attention because of their extremely long fossil record. Phycomata are known as acritarchs well into Proterozoic time. One genus (Tasmanites)  dates back to 600 Mya. Javaux et al. (2004) have turned up an entire menagerie of forms from the Mesoproterozoic, and even beyond (at least 1500 Mya), which are almost certainly eukaryotic and could well be prasinophytes, or somewhat stemward of the plants. They cannot be too distantly related, as the presence of thick organic walls, with extreme resistance to degradation, seems to be a trait of the plant-chromist lineage. One of these in particular, Leiosphaeridia crassa, from the c. 1460 Mya Roper Fm. of northern Australia, is being investigated as a possible green alga. Interestingly, in Recent or merely Paleozoic forms, these relatively large, thick-walled morphs are associated with moderately anoxic conditions and nutrient exhaustion during algal blooms.

Image: Halosphaera viridis from the National Oceanographic Data Center.

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Within the Chlorobionta are two large clades making up the "green algae." The green algae, as currently conceived, have no formal taxonomic name. We will define the group as Quercus + Chlamydomonas. The corresponding stem clades are Chlorophyta (Chlamydomonas > Quercus) and Charophyta (Quercus > Chlamydomonas).  "Chlorophyta" is also the old name for all green algae, so this is perhaps unnecessarily confusing. Tough luck. The ambiguity is now so embedded in the literature that there's nothing anyone can do about it.

The Chlorophyta have largely been delineated by molecular techniques, so it is a bit difficult to describe their characters. We know of two possible synapomorphies of the Chlorophyta. First, chlorophyte sexual forms bear paired apical flagellae usually separated by 180°, but sometimes at the same end. Second, they retain the nuclear envelope during mitosis. Indeed, chlorophytes seem to be distinguished by a variety of bizarre variations on the usually pedestrian theme of mitosis; however those variations are not entirely consistent within the group. 

Like the land plant lineage, they tend to form large aggregates, with some tissue differentiation (primarily holdfasts and reproductive structures). They are very often found in terrestrial and fresh water environments, with a distinct preference for very cold environments, such as under snow cover, or even within Antarctic ice.  Various species are important in forming symbiotic relationships with fungi, i.e., lichens. As with all green algae, chlorophytes tend to have a double cell wall -- an inner wall of cellulose and an outer gelatinous wall of protein, particularly pectin, known in higher plants as a marker for parenchyma. Starch stored in pyrenoids, located inside the chloroplasts.

Image: Ulva lobata from the California Biota Website.

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The Charophyta are the other lineage of green algae, the group which includes the land plants. Karol et al. (2001). As mentioned above, our working definition is Quercus (oak) > Chlamydomonas. The Charophyta have recently been referred to as the Streptophyta, but the reasons given for this change in nomenclature are probably insufficient. Unfortunately, the name is also frequently, and wrongly, used in place of Charophycea or Charales to describe the stoneworts -- one of several distinct groups of charophytes.

The synapomorphies of the group are said to include the the dissolution of the nuclear membrane during mitosis and the presence of paired flagella (when flagella are present at all) directed perpendicularly to each other. In addition, the charophytes are strongly inclined toward growth as long filaments.

Image: Klebsormidium from The Delwiche Lab at the Univ. of Maryland.

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The Embryophyta constitute the terrestrial or land plants, the first representatives of which appeared during the Silurian or possibly even the Middle or Late Ordovician period. The most primitive of these are nonvascular land plants, a group that classically includes liverworts (Hepatophyta / Hepaticopsida), hornworts (Anthocerotophyta / Antheroceratopsida) and mosses Bryophyta). The majority of land plants however are included within the huge and diverse clade awkwardly named Tracheophyta, including the Vascular Pants, and a basal radiation of plants loosely called rhyniophytes.

We treat Embryophyta in a specialized sense, as Quercus + moss. This may be a mistake, as this definition probably excludes the liverworts (see image) and perhaps even the hornworts. Both of these groups have traditionally been thought of as embryophytes.

Embryophytes (including liverworts) have the following synapomorphies: 1) a life cycle with alternation of generations  2) apical cell growth (some kind of meristem-like growth organization), 3) cuticle needed to control water loss on land), 4) antheridia (male gametophyte organs), and 5) archegonia (female gametophyte organs). The more derived embryophytes are vascular plants. Vascular plants have an elaborate system of conducting cells, consisting of xylem - in which water and minerals are transported) and phloem (in which carbohydrates are transported). This method of internal support enables them to stand and grow upright and pull up nutrients against the force of gravity. There are two developmental grades - those that reproduce by means of spores, and hence are dependent on water or extensive moisture (e.g. ferns), and those that reproduce by means of seeds e.g. conifers and flowering plants). The most primitive forms reproduce by means of spores haploid 1N) spores). They generally require a moist environment, because the flagellated sperm require water for fertilization.

text by MA Kazlev 2002, revised ATW041215, ATW090227.

If the mosses had not survived into the present, we would be forced to invent them as just the sort of intermediate we might expect between essentially aquatic algae and fully terrestrial plants. Mosses do have differentiated stems. Although these are generally only a few millimeters tall, they are still designed to provide mechanical support against gravity without help from water -- the first such structure in any kingdom. Bryophytes also have leaves. These are typically one cell thick and lack veins, although they may have a central thickening for support. Mosses also have rhizomes. These may have some function in extracting soil nutrients, although their primary function seems to be mechanical attachment to the substrate. Thus they are not true roots, but do approach that condition.

The bottom line is that, structurally, mosses really differ from rhyniophytes in only one aspect: mosses lack specialized vascular tissues. That alone is sufficient to explain the lack of big leaves, long stems, and true roots. This whole complex of characters is thus probably primitive. The other distinctive character of mosses is that the plant we normally observe is the haploid, gametophyte stage. But this character is shared with liverworts (basal embryophytes) and so is also probably plesiomorphic.

Curiously, in hornworts (also basal embryophytes) the sporophyte generation is dominant. In addition, it turns out that the leaves of moss probably evolved independently from the leaves of higher plants. So the relationships of the mosses and basal embryophytes are still uncertain. What really does seem to set mosses apart is their unique form of leaf. What really seems to unite mosses with higher plants is (a) the presence of stomata to control water loss and (b) meristem apical growth) in the sporophyte generation. See, Friedman et al. (2004).  Phylogenetically, we treat Bryophyta as Moss > Quercus.

Image: adapted from BIODIDAC.

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See Tracheophyta. That section covers the basal rhyniophytes, such as Horneophyton, which were the first real land plants. These probably evolved in the Ludlow and formed the stem group for all other land plants. Consequently, they are paraphyletic. The working phylogenetic definition is definition is Quercus > moss. Cantino et al. (2007).

This group is characterized by the ability to reproduce without open water. Anatomically, in all tracheophytes, the (diploid) sporophyte generation is dominant, and the sporophyte is branched. For this reason, these plants are often referred to as polysporangiophytes. In addition, the archegonium develops inside the body of the plant, rather than being superficial as in mosses and most basal embryophytes. Kenrick & Crane (1997).

Horneophyton and a few other basal forms lack tracheids. That is, they are avascular plants. However, almost all other tracheophytes have some development of specialized vascular tissues. The most basal tracheid type, present in most stem tracheophytes, appears to be the S-type tracheid.

Image: Horneophyton sculpture by Stephen Caine for the Rhynie Research Group, University of Aberdeen. Image substantially modified by ATW041229.

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The Lycophyta include the lycopsids, zosterophylls, and related forms, including (probably) a number of plants often treated as basal rhyniophytes, such as Baragwanathia. Kenrick & Crane (1997). Since they are a complex group and are treated extensively elsewhere, we will defer discussion to a revision of the existing materials.

The clade that unites oak trees and ferns is Euphyllophyta = Quercus + Equisetum.  The two complementary stem clades are Monilophyta and Spermatophytata.   Euphyllophytes are characterized (Kenrick & Crane, 1997) by monopodial or pseudomonopodial branching, helical arrangement of branches, small, pinnule-like vegetative branches, the branch apex is recurved or coiled, paired sporangia which split open along one side through a single slit, and radially-alligned xylem in the larger axes. Only early Euphyllophytes have P-type tracheids. Kenrick & Crane identified this clade based entirely on morphological characters. However, Euphytophytina has also been recovered, with essentially the same structure, using ssu rDNA. Duff & Nickrent (1999).

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The Monilophyta are the horsetails and ferns, including the Psilotidae (whisk ferns). They are closely related to the seed plants. Pryer et al. (2001). So, for example, they exhibit apical growth (meristem) in both sporophyte and gametophyte generations. They have well-developed roots megaphyllous leaves and the vascular system needed to make use of both. However, both may have been evolved independently of higher plants. Friedman et al. (2004). In addition, Monilophyta lack a complete vascular cambium, and growth of xylem is restricted to lobes of the primary xylem strand.

Since this is a new clade -- discovered, for all practical purposes, by Preyer's group, we have little to say about Monilophyta as a taxon, and defer discussion to a fuller consideration of its three component parts. The Psilotidae are the most basal, followed by the horsetails, then the remainder of the ferns.

We apply a crown group defiition to Monilophyta: Equisetum + ferns.

Image: Psilotum nudum from the University of Wisconsin - Madison Plant Systematics Collection.

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The clade that unites oaks and lycopsids is Eutracheophyta. The two complementary stem clades are Lycophyta and Spermatophytata = Quercus > Lepidodendron.   A second way to look at Spermatophytata is as the stem group leading to angiosperms. It includes Trimerophyta and the progymnosperms, in fact everything up to and including the seed plants (Spermatopsida). However, we will only be concerned with the more basal forms for now.  A third way of considering Spermatophytata is as the seed plants. However, this applies only to living forms. The basal Trimerophyta and their immediate descendants (assuming Trimerophyta is paraphyletic) lacked seeds, true leaves, or even, perhaps, roots. It is quite likely that virtually all the important land plant adaptations were independently developed in the monilophyte and spermatophytate lineages.

What seems to have set Spermatophytata apart quite early is not, in fact, the development of seeds, but the evolution of a full vascular cambium which permitted secondary growth.   Early plants with apical growth were able to use that trait to grow taller and (a) get more sunlight b) shade their competition and (c) have a better shot at spore dispersal. However, supporting a long stalk is much easier with a wider central column. Less derived groups either had no way to do this, or developed lateral lobes of the apical meristem. The latter worked, but required the tree to grow wide before it grew tall. The evolution of a complete vascular cambium permitted the tree to grow just wide enough to suit its height -- growing continuously wider as it grew tall.

The evolution of seeds follwed this innovation.   Seeds are embryonic sporophytes, held in a sort of metabolic stasis and provided with enough food to get started once their growth has been re-stared by exposure to suitable growing conditions. Well adapted seeds combined sexual reproduction with spore-like wide dispersal and so made the alternation of generations obsolete. However, early seeds, which might lack these refinements, probably evolved on tall trees which gave any sort of propagule a head start in dispersal.

Image: Psilophyton from the PhD project page of Ben Sheppard at the University of Sheffield.

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